\documentclass[a4paper]{article}

\usepackage{pgfplots}

\pgfplotsset{compat=1.5}

\begin{document}

2cm: \hrule width 2cm

\begin{tikzpicture}
	\begin{axis}[
		title={x: abs=2cm},
		enlarge x limits={abs=2cm},
	]
	\addplot+[domain=-6:6,samples=2] {x};
	\end{axis}
\end{tikzpicture}
\end{document}

